1. Field of the Invention
The present invention relates to an ND filter whose transmittance is reduced more in the central portion than in the peripheral portion.
2. Description of the Related Art
For instance, in a lens for a CCTV, a concentric ND filter having concentric filter elements is used to obtain an apparent F-number larger than the maximum F-number which is determined by the minimum size of the mechanical diaphragm aperture by reducing the quantity of light passing through the filter. FIG. 22 shows a conceptual view of a concentric ND filter, in which a plurality of multi-layered circular ND filter elements (light absorbing layers) 12, 13, 14 whose diameters are reduced stepwise in this order are formed in a concentrical arrangement on a transparent substrate 11 by an evaporation process. In general, the thickness of the transparent substrate 11 is approximately 0.5 mm and the thickness of each circular filter layer 12, 13 or 14 is in the order of a few tenths of a micrometer. In the concentric ND filter 10 shown in FIG. 22, since the quantity of light passing therethrough decreases as the mechanical diaphragm (stop) becomes smaller, a large apparent F-number can be obtained.
However, it has been found in a lens having the concentric ND filter 10 that a slight focus shift occurs between in a fully-open position of the mechanical stop and in the minimum stop position, thus leading to a deterioration of the image quality.